1. Field of the Invention
The present invention relates to a monopole antenna that is used for transmission and reception of satellite communication and satellite broadcast. In particular, the invention relates to a monopole antenna that is suitable for vehicular and portable use.
2. Description of the Related Art
A rod-shaped radiation conductor that erects from a ground surface of a metal plate or the like and that has an overall length of xcex/4 (xcex: the free space wavelength of radio waves) is widely employed as a monopole antenna that is used in mobile communication equipment etc. for transmission and reception of radio waves in a frequency range of 800-2,000 MHz. In such a monopole antenna, a feeder line such as a coaxial cable is connected to the bottom end of the radiation conductor that extends in the vertical direction. The length of the radiation conductor is so set that the radiation conductor resonates with radio waves having a desired frequency.
In vehicular telephones etc., a dual-band monopole antenna that can be used for transmission and reception of both of radio waves of an 800-MHz frequency band and radio waves of 1.9-GHz frequency band, for example, is required. Conventionally, two rod-shaped radiation conductors that erect from a ground surface of a metal plate or the like and that extend in the vertical direction are widely employed in this kind of dual-band monopole antenna. Since the overall length of each of the two radiation conductors is set to xcex/4 (xcex: the free space wavelength of corresponding radio waves), one rod-shaped radiation conductor for transmission and reception of radio waves of a lower frequency band is long and the other rod-shaped radiation conductor for transmission and reception of radio waves of a higher frequency band is short. Feeder lines such as coaxial cables are connected to the bottom ends of the two rod-shaped radiation conductors, respectively, whereby signals having different frequencies are supplied to the respective radiation conductors.
However, in the above-described conventional monopole antenna, the overall length of the rod-shaped radiation conductor is equal to xcex/4. Therefore, to transmit and receive radio waves of the 800-MHz band which is frequently used for cellular phones, for example, a radiation conductor whose overall length almost amounts to 10 cm is necessary. This means a problem that the height dimension is too large for use as a vehicular monopole antenna. In addition, this kind of monopole antenna has a narrow resonance frequency band, that is, it resonates with only radio waves whose frequency is close to a particular frequency. This raises fear that the sensitivity may decrease extremely when ratio waves to be received are deviated in frequency.
In view of the above, recently, a monopole antenna has been proposed that is reduced in height dimension by forming a band-shaped radiation conductor having a constant width on the surface of a dielectric substrate made of ceramics or the like by printing, etching, or a like method. According to this conventional technique, the overall length of the radiation conductor can be reduced by about 20% by virtue of the wavelength shortening by the dielectric. However, where the height dimension is restricted severely as in the case of monopole antennas for vehicular use, it is desired that the radiation conductor be shortened further.
The present invention has been made in view of the above circumstances in the art, and an object of the invention is therefore to provide a monopole antenna that can easily be reduced in height dimension and hence can accelerate miniaturization.
To attain the above object, a first aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground surface; and a radiation conductor that is provided on a surface of the dielectric substrate so as to extend in the vertical direction, a bottom end of the radiation conductor being connected to a feeder line, the radiation conductor having a bottom portion and a top portion that is distant from the ground surface and is wider than the bottom portion.
In the monopole antenna having the above configuration, since the top portion (capacitive region), having a large voltage variation, of the radiation conductor is wide, the capacitance is increased. In general, the resonance frequency of a resonance circuit lowers as its capacitance increases. Therefore, if the length of the radiation conductor is equivalent, the resonance frequency of this monopole antenna is lower than that of a monopole antenna in which a band-shaped radiation conductor not having a wide top portion is formed on the surface of a dielectric substrate. That is, providing the wide portion as the top portion makes it possible to set short a length of the radiation conductor that is necessary to attain resonance at a desired frequency and hence to reduce the height dimension of the entire monopole antenna easily.
In the above configuration, the dielectric substrate may be formed with through-holes or thin portions in a bottom region. In this case, the inductance increases because the dielectric constant decreases around the bottom portion (inductive region) of the radiation conductor. In general, the resonance frequency of a resonance circuit lowers as its inductance increases. Therefore, in this case, a length of the radiation conductor that is necessary to attain resonance at a desired frequency can be set shorter and hence the height dimension of the entire monopole antenna can further be reduced. A ground electrode may be provided on the dielectric substrate in a bottom end region and soldered to the ground surface. This makes it unnecessary to screw the dielectric substrate to the ground surface, which makes work of connecting the monopole antenna to a feeder line such as a coaxial cable easier.
A second aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground surface; a first radiation conductor that is provided on a surface of the dielectric substrate so as to extend in the vertical direction, a bottom end of the first radiation conductor being connected to a feeder line; and a second radiation conductor that extends parallel with a plane that is approximately perpendicular to the dielectric substrate, the second radiation conductor being connected to a top end of the first radiation conductor.
In the monopole antenna having the above configuration, a maximum voltage variation occurs in the second radiation conductor which is connected to the top end of the first radiation conductor. Since the second radiation conductor is extended to a plane that is approximately perpendicular to the dielectric substrate, the capacitance is large there. In general, the resonance frequency of a resonance circuit lowers as its capacitance increases. Therefore, if the overall height dimension is equivalent, the resonance frequency of this monopole antenna is lower than that of a conventional one in which only a band-shaped radiation conductor having a constant width is formed on the surface of a dielectric substrate. Therefore, in this monopole antenna, an overall height dimension for attaining resonance at a desired frequency can be made shorter than in such a conventional monopole antenna.
In the above configuration, the second radiation conductor may be a metal plate. Alternatively, a small dielectric substrate may be provided on the dielectric substrate so as to be approximately perpendicular to the dielectric substrate, and the second radiation conductor may be provided on one or both surfaces of the small dielectric substrate. In this case, in manufacture, the first and second radiation conductors can be formed together on the surfaces of the dielectric substrate and the small dielectric substrate that are formed from a common substrate, which is suitable for mass-production. In addition, the resonance frequency can further be lowered by utilizing the wavelength shortening effect of the small dielectric substrate.
In each of the above configurations according to the second aspect of the invention, the first radiation conductor may have a bottom portion and a top portion (capacitive region) that is distant from the ground surface and is wider than the bottom portion. In this case, the capacitance of the first radiation conductor increases and hence the resonance frequency further lowers. Therefore, the height dimension of the entire monopole antenna can further be reduced.
In each of the above configurations according to the second aspect of the invention, the dielectric substrate may be formed with through-holes or thin portions in a bottom region. In this case, the inductance increases because the dielectric constant decreases around the bottom portion (inductive region) of the radiation conductor. In general, the resonance frequency of a resonance circuit lowers as its inductance increases. Therefore, in this case, a length of the radiation conductor that is necessary to attain resonance at a desired frequency can be set shorter and hence the height dimension of the entire monopole antenna can further be reduced. A ground electrode may be provided on the dielectric substrate in a bottom end region and soldered to the ground surface. This makes it unnecessary to screw the dielectric substrate to the ground surface, which makes work of connecting the monopole antenna to a feeder line such as a coaxial cable easier.
A third aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground surface; a first radiation conductor that is provided on a surface of the dielectric substrate so as to extend in the vertical direction and that has a feeding point at a bottom end; and a second radiation conductor that is provided on a surface of the dielectric substrate so as to have approximately the same shape as the first radiation conductor and to have a parallel positional relationship with the first radiation conductor, and that has a feeding point at a bottom end, wherein the first and second radiation conductors have different lengths and signals having the same frequency are supplied to the feeding points of the first and second radiation conductors, respectively.
In the monopole antenna having the above configuration, by coupling appropriately together the first radiation conductor and the second radiation conductor that are slightly different from each other in length by using a capacitor or the like, the monopole antenna can resonate with both of radio waves whose wavelength corresponds to the length of the first radiation conductor and radio waves whose wavelength corresponds to the length of the second radiation conductor, whereby the resonance frequency band can be widened to a large extent. Since the first and second radiation conductors are formed on the surface of the dielectric substrate made of ceramics or the like, the length of each radiation conductor can be set with an additional effect of wavelength shortening by the dielectric. Therefore, the height dimension of the monopole antenna can easily be reduced.
In the above configuration, the first radiation conductor may be provided on one surface of the dielectric substrate and the second radiation conductor may be provided on the opposite surface of the dielectric substrate. This allows each radiation conductor to be designed easily so as to have a desired shape. For example, each of the first and second radiation conductors may be so designed as to have a wide top portion (capacitive region) that is distant from the ground surface so that the capacitance of each radiation conductor is increased. Since the resonance frequency lowers accordingly, a length (height dimension) of each radiation conductor that is necessary to attain resonance at a desired frequency can further be reduced.
There may be provided a third radiation conductor that is provided on the dielectric substrate so as to extend parallel with a plane that is approximately perpendicular to the dielectric substrate, the third radiation conductor being connected to the top end of the first radiation conductor; and a fourth radiation conductor that is provided on the dielectric substrate so as to extend parallel with a plane that is approximately perpendicular to the dielectric substrate, the fourth radiation conductor being connected to the top end of the second radiation conductor. With this structure, the capacitance of the first and third radiation conductors as an integrated radiation conductor and the capacitance of the second and fourth radiation conductors as another integrated radiation conductor are large, whereby the resonance frequency can be lowered and the height dimension can be reduced. In this case, a small dielectric substrate may be provided on the dielectric substrate so as to be approximately perpendicular to the dielectric substrate, and the third and fourth radiation conductors may be provided on a surface of the small dielectric substrate. This makes it possible to further reduce the height dimension by utilizing the wavelength shortening effect of the small dielectric substrate.
A fourth aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground surface; a first radiation conductor that is provided on a surface of the dielectric substrate so as to extend in the vertical direction and to have a wide top portion; and a second radiation conductor that is provided on the surface of the dielectric substrate so as to extend in the vertical direction and to have a smaller length dimension than the first radiation conductor, wherein a first high-frequency signal is supplied to the first radiation conductor via a feeding point that is provided at a bottom end of the first radiation conductor and a second high-frequency signal having a higher frequency than the first high-frequency signal is supplied to the second radiation conductor via a feeding point that is provided at a bottom end of the second radiation conductor.
In the above dual-band monopole antenna, the first radiation conductor needs to be longer than the second radiation conductor because the former is lower in resonance frequency than the latter. The first radiation conductor has a large capacitance because the wide portion is formed as the top portion (capacitive region) that is distant from the ground surface. In general, the resonance frequency of a resonance circuit lowers as its capacitance increases. This monopole antenna is also given the wavelength shortening effect of the dielectric substrate. Consequently, a length of the first radiation conductor that is necessary to attain resonance at a desired frequency (of the first high-frequency signal) can be reduced to a large extent and the reduction of the height dimension of the entire monopole antenna can be accelerated. In this case, a third radiation conductor that has approximately the same shape as and a different length dimension in the vertical direction than the first radiation conductor that is provided on one surface of the dielectric substrate may be provided on the opposite surface of the dielectric substrate, and the first high-frequency signal may be supplied to the bottom end of the third radiation conductor. The resonance frequency band can be widened by coupling appropriately the first and third radiation conductors to each other by using a capacitor or the like.
The fourth aspect of the invention also provides a monopole antenna in which the first radiation conductor has an erect portion that is provided on a surface of the dielectric substrate so as to extend in the vertical direction and a horizontal portion that is provided on the dielectric substrate so as to extend horizontally and is connected to the top end of the erect portion. Also in this case, the first radiation conductor has a large capacitance. Therefore, a length of the first radiation conductor that is necessary to attain resonance at a desired frequency can be reduced. In this case, a small dielectric substrate may be provided on the dielectric substrate so as to be approximately perpendicular to the dielectric substrate, and the horizontal portion of the first radiation conductor may be provided on a surface of the small dielectric substrate. This makes it possible to further reduce the height dimension by virtue of the wavelength shortening effect of the small dielectric substrate. In this configuration, the erect portion of the first radiation conductor may have a wide top portion. This further increases the capacitance, whereby the resonance frequency can further be lowered and the reduction of the height dimension can be accelerated.
In each of the configurations according to the fourth aspect of the invention, a fourth radiation conductor that has approximately the same shape as and a different length dimension in the vertical direction than the second radiation conductor that is provided on one surface of the dielectric substrate may be provided on the opposite surface of the dielectric substrate, and the second high-frequency signal may be supplied to the bottom end of the fourth radiation conductor. The resonance frequency band can be widened by coupling appropriately the second and fourth radiation conductors to each other by using a capacitor or the like.
A branching circuit that passes signals having particular frequencies may be incorporated so that a signal having a lower frequency is supplied via a coil and a signal having a higher frequency is supplied via a capacitor. This enables common use of an input voltage source. That is, the circuit configuration can be simplified by supplying the first high-frequency signal to the first radiation conductor and the second high-frequency signal to the second radiation conductor from a common input voltage source via the coil and the capacitor, respectively.
A fifth aspect of the invention provides a monopole antenna comprising a dielectric substrate that erects from a ground conductor; and a radiation conductor that is provided on a surface of the dielectric substrate, a bottom end of the radiation conductor being connected to a feeder line, the radiation conductor having a zigzagged band-shaped portion that extends in the vertical direction as a whole while its actual extension direction varies successively or continuously. It is preferable that the zigzagged band-shaped portion be shaped in such a manner that its actual extension direction varies in one of a crank form, a saw-tooth form, and a wave form.
Providing the radiation conductor with the zigzagged band-shaped portion makes it possible to increase its length without changing its height, which enables resonance with radio waves having a longer wavelength, that is, lowers the resonance frequency. Therefore, a height of the radiation conductor that is necessary to attain resonance at a desired frequency can be reduced. Also with the wavelength shortening effect of the dielectric substrate, the height dimension of the monopole antenna can be reduced to a large extent.
In the above configuration, the radiation conductor may have, as a top portion (capacitive region) where a large voltage variation occurs, a wide portion that is wider than the zigzagged band-shaped portion. This can increase the capacitance. In general, the resonance frequency of a resonance circuit lowers as the capacitance increases. Therefore, in this case, a height of the radiation conductor that is necessary to attain resonance at a desired frequency can further be reduced.
Alternatively, in the above configuration, there may be provided a second radiation conductor that extends parallel with a plane that is approximately perpendicular to the dielectric substrate, the second radiation conductor being connected to the top end of the radiation conductor. This can also increase the capacitance and hence can lower the resonance frequency, which enables height reduction of the radiation conductor. In this case, a small dielectric substrate may be provided on the dielectric substrate so as to be approximately perpendicular to the dielectric substrate, and the second radiation conductor may be provided on one or both surfaces of the small dielectric substrate. The resonance frequency can further be lowered by utilizing the wavelength shortening effect of the small dielectric substrate.